Cellulose fibers with improved elongation at break, and methods for producing same

ABSTRACT

The present invention provides a fiber made of cellulose formate which exhibits high tenacity and modulus properties, combined with improved values of elongation at break and of energy at break. The elongation at break, in particular, is greater than 6%. The invention also provides a method of producing the fiber by spinning a liquid crystal solution of cellulose formate according to the so-called dry-jet-wet spinning method, the coagulation stage and the neutral washing stage which follow both being carried out in acetone.

BACKGROUND OF THE INVENTION

The invention relates to fibers made of cellulose derivatives and tofibers made of cellulose regenerated from these derivatives.

"Cellulose derivatives" is here understood to mean, in a known way, thecompounds formed, as a result of chemical reactions, by substitution ofthe hydroxyl groups of cellulose, these derivatives also being known assubstitution derivatives. "Regenerated cellulose" is understood to meana cellulose obtained by a regeneration treatment carried out on acellulose derivative.

The invention more particularly relates to fibers made of celluloseformate and to fibers made of cellulose regenerated from this formate,and to the methods for producing such fibers.

Fibers made of cellulose formate and fibers made of celluloseregenerated from this formate have been described in particular inInternational Patent Application WO 85/05115 (PCT/CH85/00065), filed bythe Applicant Company, or in the equivalent Patents EP-B-179,822 andU.S. Pat. No. 4,839,113. These documents describe the production ofspinning solutions based on cellulose formate by reaction of cellulosewith formic acid and phosphoric acid. These solutions are opticallyanisotropic, that is to say that they exhibit a liquid crystal state.These documents also describe the cellulose formate fibers obtained byspinning these solutions, according to the so-called dry-jet-wetspinning technique, and the cellulose fibers obtained after aregeneration treatment of these formate fibers.

In comparison with conventional cellulose fibers, such as rayon orviscose fibers, or with other conventional non-cellulose fibers, such asnylon or polyester fibers, for example, all spun from opticallyisotropic liquids, the cellulose fibers of Application WO 85/05115 arecharacterized by a much more orderly structure, due to the liquidcrystal nature of the spinning solutions from which they emerge. Theythus exhibit very high mechanical properties in extension, in particularvery high tenacity and modulus values, but, on the other hand, arecharacterized by rather low values of elongation at break, these valuesbeing on average between 3% and 4% and not exceeding 4.5%.

However, greater values of elongation at break may be desirable whensuch fibers are used in certain technical applications, in particular ascomponents for reinforcing a tire, in particular a tire carcass casing.

SUMMARY OF THE INVENTION

The first aim of the invention is to provide fibers made of celluloseformate and fibers made of regenerated cellulose which, in comparisonwith the fibers of Application WO 85/05115, exhibit a significantlyimproved elongation at break and high properties of energy at break.

The second aim of the invention is to produce the above improvementswithout decreasing the tenacity of the fibers, which is a majoradvantage of the invention.

Another aim of the invention is to produce fibers made of regeneratedcellulose, from cellulose formate, the resistance to fatigue of which,in particular with respect to tires, is substantially improved incomparison with that of the fibers made of regenerated cellulose of theabove-mentioned Application WO 85/05115.

The fiber made of cellulose formate of the invention is characterized bythe following relationships:

Ds≧2;

Te>45;

Mi>800;

ELb>6;

Eb>13.5,

Ds being the degree of substitution of the cellulose as formate groups(in %), Te being its tenacity in cN/tex, Mi being its initial modulus incN/tex, ELb being its elongation at break in % and Eb being its energyat break in J/g.

The fiber made of cellulose of the invention, regenerated from celluloseformate, is characterized by the following relationships:

0<Ds<2;

T_(E) >60;

M_(I) >1000;

EL_(B) >6;

E_(B) >17.5,

D_(s) being the degree of substitution of the cellulose as formategroups (in %), T_(E) being its tenacity in cN/tex, M_(I) being itsinitial modulus in cN/tex, EL_(B) being its elongation at break in % andE_(B) being its energy at break in J/g.

The fiber made of cellulose formate and the fiber made of regeneratedcellulose above are both obtained by virtue of novel and specificmethods which constitute other subjects of the invention.

The spinning method of the invention, in order to obtain the fiber madeof cellulose formats of the invention, which consists in spinning asolution of cellulose formate in a solvent based on phosphoric acid,according to the so-called dry-jet-wet spinning method, is characterizedin that the stage of coagulation of the fiber and the stage of neutralwashing of the coagulated fiber are both carried out in acetone.

The regeneration method of the invention, in order to obtain the fibermade of regenerated cellulose of the invention, which consists inpassing a fiber made of cellulose formate into a regenerating medium, inwashing it and then in drying it, is characterized in that theregenerating medium is an aqueous sodium hydroxide (NaOH) solution inwhich the sodium hydroxide concentration, recorded as Cs, is greaterthan 16% (% by weight).

The invention additionally relates to the following products:

reinforcing assemblies each containing at least one fiber in accordancewith the invention, for example cables, plied yarns or multifilamentfibers twisted on themselves, it being possible for such reinforcingassemblies to be, for example, hybrids, that is to say composites,containing components of different natures, optionally not in accordancewith the invention;

articles reinforced by at least one fiber and/or one assembly inaccordance with the invention, these articles being, for example, rubberor plastic articles, for example plies, belts, pipes or tires, inparticular tire carcass casings.

The invention will easily be understood with the help of the descriptionand the non-limiting examples which follow.

DESCRIPTION OF PREFERRED EMBODIMENTS I. MEASUREMENTS AND TESTS USED

I-1. Degree of Polymerization

The degree of polymerization is recorded as DP. The DP of cellulose ismeasured in a known way, this cellulose being in powder form orconverted beforehand to powder.

The inherent viscosity (IV) of the dissolved cellulose is first of alldetermined according to Swiss Standard SNV 195 598 of 1970, but atdifferent concentrations which vary between 0.5 and 0.05 g/dl. Theinherent viscosity is defined by the equation:

    IV=(I/C.sub.e)×Ln (t.sub.1 /t.sub.0)

in which C_(e) represents the concentration of dry cellulose, t₁represents the duration of flow of the dilute polymer solution, t₀represents the duration of flow of the pure solvent, in a Ubbelhode-typeviscometer, and Ln represents the Naperian logarithm. The measurementsare taken at 20° C.

The intrinsic viscosity [η] is then determined by extrapolation of theinherent viscosity IV to zero concentration.

The weight-average molecular mass M_(w) is given by the Mark-Houwinkrelationship:

    [η]=K×M.sub.w.sup.α

where the constants K and α are respectively:

K=5.31×10⁻⁴ ; α=0.78, these constants corresponding to the solventsystem used to determine the inherent viscosity. These values are givenby L. Valtasaari in the document Tappi 48, 627 (1965).

The DP is finally calculated according to the formula:

    DP=(M.sub.w)/162,

162 being the molecular mass of the elementary cellulose unit.

When it is a matter of determining the DP of cellulose from celluloseformate in solution, this formate must first of all be isolated and thenthe cellulose regenerated.

The procedure is then as follows:

the solution is first of all coagulated with water in a dispersingdevice. After filtration and washing with acetone, a powder is obtainedwhich is subsequently dried in an oven under vacuum at 40° C. for atleast 30 minutes. After having isolated the formate, the cellulose isregenerated by treating this formate at reflux with normal sodiumhydroxide solution. The cellulose obtained is washed with water anddried and the DP is measured as described above.

I-2. Degree of Substitution

The degree of substitution of cellulose as cellulose formate is alsoknown as degree of formylation.

The degree of substitution determined by the method described here givesthe percentage of alcohol functional groups in the cellulose which areesterified, that is to say converted to formate groups. This means thata degree of substitution of 100% is obtained if the three alcoholfunctional groups in the cellulose unit are all esterified, or that adegree of substitution of 30%, for example, is obtained if 0.9 alcoholfunctional group out of three, on average, is esterified.

The degree of substitution is measured differently depending on whetherthe characterization is performed on cellulose formate (formate insolution or fibers made of formate) or on fibers made of celluloseregenerated from cellulose formate.

I-2.1. Degree of Substitution on Cellulose Formate:

If the degree of substitution is measured on cellulose formate insolution, this formate is first of all isolated from the solution asindicated above in paragraph I-1. If it is measured on fibers made offormate, these fibers are precut into pieces 2 to 3 cm long.

200 mg of cellulose formate thus prepared are weighed out accurately andintroduced into a conical flask. 40 ml of water and 2 ml of normalsodium hydroxide solution (1N NaOH) are added. The mixture is heated at90° C. at reflux for 15 minutes under nitrogen. The cellulose is thusregenerated, the formate groups being reconverted to hydroxyl groups.After cooling, the excess sodium hydroxide is back titrated with adecinormal hydrochloric acid solution (0.1N HCl) and the degree ofsubstitution is thus deduced therefrom.

In the present description, the degree of substitution is recorded as Dswhen it is measured on fibers made of cellulose formate.

I-2.2. Degree of Substitution on Fibers Made of Regenerated Cellulose:

Approximately 400 mg of fiber are cut into pieces 2 to 3 cm along, thenweighed accurately and introduced into a 100 ml conical flask containing50 ml of water. 1 ml of normal sodium hydroxide solution (1N NaOH) isadded. The components are mixed at room temperature for 15 minutes. Thecellulose is thus regenerated completely by converting, to hydroxylgroups, the final formate groups which had withstood the regenerationcarried out, after spinning them, directly on continuous fibers. Theexcess sodium hydroxide is titrated with a decinormal hydrochloric acidsolution (0.1N HCl) and the degree of substitution is thus deducedtherefrom.

In the present description, the degree of substitution is recorded asD_(s) when it is measured on fibers made of regenerated cellulose.

I-3. Optical Properties of the Solutions

The optical isotropy or anisotropy of the solutions is determined byplacing a drop of test solution between the linear crossed polarizer andanalyzer of an optical polarization microscope, followed by observingthis solution at rest, that is to say in the absence of a dynamicconstraint, at room temperature.

In a known way, an optically anisotropic solution is a solution whichdepolarizes light, that is to say which exhibits, thus placed betweenlinear crossed polarizer and analyzer, light transmission (coloredtexture). An optically isotropic solution is a solution which, under thesame observation conditions, does not exhibit the above depolarizationproperty, the field of the microscope remaining black.

I-4. Mechanical Properties of the Fibers

"Fibers" is understood here to mean multifilament fibers (also known as"spun yarns") composed, in a known way, of a large number of individualfilaments with a small diameter (low yarn count). All the mechanicalproperties below are measured on fibers which have been subjected to apreconditioning. "Preconditioning" is understood to mean the storage ofthe fibers for at least 24 hours, before measurement, in a standardatmosphere according to European Standard DIN EN 20139 (temperature of20±2° C.; hygrometry of 65±2%).

For cellulose fibers, such a preconditioning makes it possible, in aknown way, to stabilize their degree of moisture (residual watercontent) at a natural equilibrium level of less than 15% by weight ofdry fiber (approximately 11 to 12%, on average).

The yarn count of the fibers is determined on at least three samples,each corresponding to a length of 50 m, by weighing this length offiber. The yarn count is given in tex (weight in grams of 1000 m offiber).

The mechanical properties of the fibers (tenacity, initial modulus,elongation and energy at break) are measured in a known way using aZwick GmbH & Co (Germany) 1435-type or 1445-type tension machine. Thefibers, after having received a slight prior protective twist (helicalangle of approximately 6°), are subjected to tension over an initiallength of 400 mm at a rate of 200 mm/min (or at a rate of 50 mm/min onlywhen their elongation at break does not exceed 5%). All the resultsgiven are an average of 10 measurements.

The tenacity (breaking strength divided by the yarn count) and theinitial modulus are indicated in cN/tex (centinewton per tex--reminder:1 cN/tex equals approximately 0.11 g/den (gram per denier)). The initialmodulus is defined as the slope of the linear part of theForce-Elongation curve, which occurs just after the standard 0.5 cN/texpretension. The elongation at break is indicated as a percentage. Theenergy at break is given in J/g (joule per gram), that is to say perunit of fiber mass.

II. CONDITIONS FOR IMPLEMENTING THE INVENTION

A description is first of all given of the preparation of the spinningsolutions, followed by the spinning of these solutions in order toproduce fibers made of cellulose formate. The stage of regeneration ofthe fibers made of cellulose formate, in order to produce fibers made ofregenerated cellulose, is explained in a third paragraph.

II-1. Preparation of the Spinning Solutions

The cellulose formate solutions are prepared by mixing cellulose, formicacid and phosphoric acid (or a liquid based on phosphoric acid) asindicated, for example, in the abovementioned Application WO 85/05115.

The cellulose can be provided in different forms, in particular in theform of a powder, prepared, for example, by pulverizing a crudecellulose plate. Its initial water content is preferably less than 10%by weight and its DP between 500 and 1000.

The formic acid is the esterification acid, the phosphoric acid (or theliquid based on phosphoric acid) being the solvent for the celluloseformate, known as "solvent" or alternatively "spinning solvent" in thedescription below. In general, the phosphoric acid used isorthophosphoric acid (H₃ PO₄) but it is possible to use other phosphoricacids or a mixture of phosphoric acids. The phosphoric acid can,depending on the situation, be used solid, in the liquid state or elsedissolved in the formic acid.

The water content of these two acids is preferably less than 5% byweight; they can be used alone or can optionally contain, in smallproportions, other organic and/or inorganic acids, such as acetic acid,sulfuric acid or hydrochloric acid, for example.

In accordance with the description given in the abovementionedApplication WO 85/05115, the cellulose concentration in the solution,recorded as "C" below, can vary to a large extent; concentrations C ofbetween 10% and 30% (% by weight of cellulose, calculated on the basisof a non-esterified cellulose, with respect to the total weight of thesolution) are possible, for example, these concentrations being inparticular a function of the degree of polymerization of the cellulose.The (formic acid/phosphoric acid) ratio by weight can also be adjustedwithin a wide range.

During the preparation of the cellulose formate, the use of formic acidand of phosphoric acid makes it possible to obtain both a high degree ofsubstitution as cellulose formate, generally greater than 20%, withoutexcessively decreasing the initial degree of polymerization of thecellulose, and a homogeneous distribution of these formate groups, bothin the amorphous regions and in the crystalline regions of the celluloseformate.

The kneading means appropriate for the production of a solution areknown to a person skilled in the art: they must be suitable forkneading, correctly mixing, preferably at an adjustable rate, thecellulose and the acids until the solution is obtained. "Solution" ishere understood to mean, in a known way, a homogeneous liquidcomposition in which no solid particle is visible to the naked eye. Thekneading can be carried out, for example, in a mixer having Z-shapedmixing arms or in a continuous screw mixer. These kneading means arepreferably equipped with a device for discharge under vacuum and with aheating and cooling device which makes it possible to adjust thetemperature of the mixer and of its contents, in order, for example, toaccelerate the dissolution operations, or to control the temperature ofthe solution during formation.

By way of example, the following procedure can be used.

Cellulose powder (the moisture content of which is in equilibrium withthe surrounding moisture content of the air) is introduced into ajacketed mixer having Z-shaped mixing arms and an extrusion screw. Amixture of orthophosphoric acid (99% crystalline) and of formic acid,for example containing three quarters of orthophosphoric acid per onequarter of formic acid (parts by weight), is subsequently added. Theentire contents are mixed for a period of approximately 1 to 2 hours,for example, the temperature of the mixture being maintained between 10and 20° C., until a solution is obtained.

The spinning solutions thus obtained are ready to be spun; they can betransferred directly, for example via an extrusion screw placed at theoutlet of the mixer, to a spinning machine in order to be spun thereon,without prior conversion other than conventional operations, such asdegassing or filtration stages, for example.

The spinning solutions used for the implementation of the invention areoptically anisotropic solutions. These spinning solutions preferablyexhibit at least one of the following characteristics:

their cellulose concentration is between 15% and 25% (% by weight),calculated on the basis of a non-esterified cellulose;

their total formic acid concentration (that is to say the formic acidpart consumed in the esterification plus the free formic acid partremaining in the final solution) is between 10 and 25% (% by weight);

their phosphoric acid concentration (or concentration of liquid based onphosphoric acid) is between 50% and 75% (% by weight);

the degree of substitution of the cellulose as formate groups in thesolution is between 25% and 50%, more preferably between 30% and 45%;

the degree of polymerization of the cellulose, in solution, is between350 and 600;

they contain less than 10% water (% by weight).

II-2. Spinning of the Solutions

The spinning solutions are spun according to the so-calleddry-jet-wet-spinning technique: this technique uses a non-coagulatingfluid layer, generally air, placed at the die outlet, between the dieand the coagulation means.

At the outlet of the kneading and dissolution means, the spinningsolution is transferred to the spinning unit where it feeds a spinningpump. From this spinning pump, the solution is extruded through at leastone die, preceded by a filter. On its way to the die, the solution isgradually brought to the desired spinning temperature, generally between35° C. and 90° C., depending on the nature of the solutions, preferablybetween 40° C. and 70° C. "Spinning temperature" is thus understood tomean the temperature of the spinning solution at the moment when it isextruded through the die.

Each die can contain a variable number of extrusion capillaries, itbeing possible for this number to vary, for example, from 50 to 1000.The capillaries are generally cylindrical in shape, it being possiblefor their diameter to vary, for example, from 50 to 80 μm (micrometers).

At the die outlet, a liquid extrudate is thus obtained which is composedof a variable number of individual liquid veins. Each individual liquidvein is drawn (see spinning-stretch factor SSF or spinning-draw factorSDF hereinbelow) into a non-coagulating fluid layer, before entering thecoagulation region. This non-coagulating fluid layer is generally alayer of gas, preferably of air, the thickness of which can vary from afew mm to several tens of mm (millimeters), for example from 5 mm to 100mm, depending on the specific spinning conditions; in a known way,thickness of the non-coagulating layer is understood to mean thedistance separating the lower face of the die, arranged horizontally,and the inlet of the coagulation region (surface of the coagulatingliquid).

After passing through the non-coagulating layer, all the liquid veinsthus drawn enter the coagulation region and come into contact with thecoagulating medium. Under the action of the latter, they are converted,by precipitation of the cellulose formate and extraction of the spinningsolvent, to solid filaments of cellulose formate which thus form afiber.

The coagulating medium employed is acetone.

The temperature of the coagulating medium, recorded as Tc, is not acritical parameter in the implementation of the invention. By way ofexample, for spinning solutions containing 22% by weight of cellulose,it has been observed that a variation in temperature Tc throughout thetemperature range from -30° C. to 0° C. has virtually no effect on themechanical properties of the fibers obtained.

A negative temperature Tc, that is to say less than 0° C., willpreferably be chosen and, in an even more preferable way, less than -10°C.

A person skilled in the art will know how to adjust the temperature ofthe coagulating medium, depending on the characteristics of the spunsolution and on the targeted mechanical properties, by simpleoptimization tests. Generally, the temperature Tc will be chosen to belower as the concentration C of the spinning solution becomes lower.

The degree of spinning solvent in the coagulating medium is preferablystabilized at a level of less than 15%, more preferably still less than10% (% by weight of coagulating medium).

The coagulation means to be employed are known devices, composed, forexample, of baths, pipes and/or chambers, containing the coagulatingmedium and in which the fiber in the course of formation moves. Use ispreferably made of a coagulation bath arranged under the die, at theoutlet of the non-coagulating layer. This bath is generally extended atits base by a vertical cylindrical tube, a so-called "spinning tube",into which the coagulated fiber passes and in which the coagulatingmedium circulates.

The depth of coagulating medium in the coagulation bath, measured fromthe inlet of the bath to the inlet of the spinning tube, can vary from afew millimeters to a few centimeters, for example, depending on thespecific conditions for implementing the invention, in particulardepending on the spinning rates used. The coagulation bath can beextended, if necessary, by additional coagulation devices, for exampleby other baths or chambers, placed at the outlet of the spinning tube,for example after a horizontal return point.

The method of the invention is preferably employed so that at least oneof the following characteristics is verified:

a) the degree of residual solvent in the fiber, at the outlet of thecoagulation means (recorded as Sr), is less than 100% by weight of dryfiber made of formate;

b) the tensile stress undergone by the fiber, at the outlet of thecoagulation means (recorded as σ_(c)), is less than 5 cN/tex,

and, in an even more preferable way, so that the two characteristics a)and b) above are simultaneously verified.

Thus, according to the above preferred conditions, the fiber is left incontact with the coagulating medium until a significant portion ofspinning solvent is extracted from the fiber. Moreover, during thiscoagulation phase, the emphasis is on maintaining the tensions undergoneby the fiber at a moderate level: to monitor this, these tensions willbe measured immediately at the outlet of the coagulation means, usingappropriate tensiometers.

Generally, if it is desired to favor, above everything else, theproperties of elongation at break of the fibers made of formate, theinvention will preferably be implemented so that the following tworelation ships are verified:

    Sr<50%; σ.sub.c <2 cN/tex.

The degree of residual solvent Sr present in the coagulated fiber madeof formate is measured, for example, in the following way: fiber iswithdrawn at the outlet of the coagulation means, with its coagulatingmedium; it is then superficially dried with an absorbent paper, withoutpressure, so as to remove most of the coagulating medium (acetone) whichis contained in the surface layer surrounding the fiber and which itselfcontains a certain fraction of spinning solvent (phosphoric acid orliquid based on phosphoric acid) already extracted from the fiber; thefiber is subsequently washed completely with water, in a laboratorydevice, so as to completely extract the phosphoric acid which itcontains, and then this phosphoric acid is back titrated with sodiumhydroxide; for greater accuracy, the measurement is repeated 5 times andthe mean is calculated.

At the outlet of the coagulation means, the fiber is taken up on a drivedevice, for example on motorized rollers. The rate of the spun producton this drive device is known as the "spinning rate" (or alternativelydelivery or take-up rate): it is the rate of progression of the fiberthrough the spinning plant, once the fiber has been formed. The ratio ofthe spinning rate to the extrusion rate of the solution through the diedefines what is known, in a known way, as the spinning-stretch factor orspinning-draw factor (abbreviated to SSF or SDF), which is, for example,between 2 and 10.

Once coagulated, the fiber must be washed to neutrality. "Neutralwashing" is understood to mean any washing operation which makes itpossible to extract all or virtually all the spinning solvent from thefiber.

A person skilled in the art was naturally, until now, directed to usingwater as washing medium: in a well known way, water is indeed the"natural" swelling medium for fibers made of cellulose or of cellulosederivatives (see, for example, U.S. Pat. No. 4,501,886) and consequentlythe medium capable of offering, a priori, the best washing efficiency.

By way of example, Patents or Patent Applications EP-B-220,642, U.S.Pat. No. 4,926,920 and WO 94/17136, like the abovementioned ApplicationWO 85/05115 (page 72, Examples II-1 et seq.), describe the use of water,at the outlet of the coagulation means, for washing fibers made ofcellulose formate.

Nevertheless, such a conventional stage of washing with water does notmake it possible to obtain fibers made of cellulose formate inaccordance with the invention.

In an entirely surprising way, it has been found that the acetoneemployed as washing medium, despite a washing power which is, in a knownway, markedly lower than that of water, results in fibers which exhibit,once completed (i.e. washed to neutrality and then dried), very markedlyimproved properties, first and foremost as regards their elongation atbreak, when they are compared with the fibers described in ApplicationWO 85/05115.

For the implementation of the method of the invention, the stage ofcoagulation of the fiber and the state of neutral washing of thecoagulated fiber must both be carried out in acetone.

The temperature of the washing acetone is not a critical parameter ofthe method. However, it is obvious that excessively low temperatureswill be avoided, so as to promote the kinetics of washing. Preferably,the temperature of the washing acetone, recorded as TW, will be chosento be positive (this is understood to mean a temperature equal to orgreater than 0° C.) and, in an even more preferable way, greater than+10° C. Advantageously, non-cooled acetone can be used, that is to sayacetone at room temperature, the washing operation then preferably beingcarried out in a controlled atmosphere.

Known washing means, for example consisting of baths containing washingacetone in which the fiber to be washed moves, can be employed. Thewashing times in acetone can typically vary from a few seconds to a fewtens of seconds, depending on the specific conditions for implementationof the invention.

Of course, the washing medium, like the coagulating medium, can bothcontain constituents other than acetone, without the spirit of theinvention being modified, provided that these other constituents areonly present in a minor proportion; the total proportion of these otherconstituents will preferably be less than 15%, more preferably less than10% (% by total weight of coagulating medium or of washing medium). Moreparticularly, if water is present in the coagulation or washing acetone,its content will preferably be less than 5%.

After washing, the fiber made of cellulose formate is dried by anysuitable means, in order to remove the washing acetone. Preferably, thedegree of acetone at the outlet of the drying means is adjusted to adegree of less than 1% by weight of dry fiber. The drying operation canbe carried out, for example, by continuous progression of the fiber overheating rollers or alternatively by employing, principally oradditionally, a technique of blowing preheated nitrogen. Preferably, useis made of a drying temperature of at least 60° C., more preferably ofbetween 60° C. and 90° C.

The method of the invention can be implemented in a very wide range ofspinning rates, which can vary from several tens to several hundreds ofmeters per minute, for example to 400 m/min or 500 m/min, if not more.Advantageously, the spinning rate is at least equal to 100 m/min, morepreferably at least equal to 200 m/min.

If it is desired to isolate the fiber made of cellulose formate, that isto say not to immediately regenerate it, in particular in order tomonitor its mechanical properties before the regeneration operations,the washing stage will preferably be carried out so that the degree ofresidual spinning solvent in the completed fiber, i.e. washed and dried,does not exceed 0.1% to 0.2% by weight with respect to the weight of dryfiber.

It is also possible to convey the fiber made of cellulose formate, thusspun, directly to the regeneration means, in line and continuously, withthe aim of preparing a fiber made of regenerated cellulose.

II-3. Regeneration of the Fibers Made of Formate

In a known way, a method for the regeneration of a fiber made ofcellulose derivative consists in treating this fiber in a regeneratingmedium so as to remove virtually all the substituent groups (so-calledsaponification treatment), in washing the thus regenerated fiber and inthen drying it, these three operations being in principle carried outcontinuously on the same treatment line, known as a "regeneration line".

As regards the cellulose formate, the regenerating medium used isgenerally a weakly concentrated aqueous sodium hydroxide (NaOH) solutioncontaining only a few percent of sodium hydroxide (% by weight), forexample from 1 to 3% (see, for example, PCT/AU91/00151).

Weakly concentrated aqueous sodium hydroxide solutions, with a sodiumhydroxide concentration not exceeding 5% (% by weight), have also beendescribed in Patents or Patent Applications EP-B-220,642, U.S. Pat. No.4,926,920, WO 94/17136 and WO 95/20629 for the regeneration of fibersmade of cellulose formate. They have been used for the regeneration ofthe fibers made of cellulose formate described in the abovementionedApplication WO 85/05115, as for the regeneration of the fibers made ofcellulose formate of the present invention; these weakly concentratedsolutions prove to be entirely satisfactory in resulting in regenerationproper, that is to say in removing virtually all the substituent formategroups: they make it possible to obtain, without difficulty, regeneratedfibers for which the degree of substitution as formate groups is lessthan 2%.

On attempting to increase the sodium hydroxide concentrations beyond 5%,the Applicant Company has found that the filaments of the fibers made ofcellulose formate (whether the latter are or are not in accordance withthe invention) underwent partial surface dissolution, as soon as thesodium hydroxide concentration reached and exceeded 6% by weightapproximately, the regenerating medium then becoming a true solvent forthe cellulose formate. Such a dissolution, even partial, is entirelyharmful to the mechanical properties of the fiber: presence of stuckfilaments, fall in strength of the filaments attacked, difficulties inwashing the fiber, and the like.

Such problems of interfering dissolution could furthermore beanticipated, it being known, for example, that cellulose fibers of theviscose type are partially or completely soluble in 10% sodium hydroxidesolution (see P. H. Hermans, "Physics and Chemistry of CelluloseFibers", 1st part, Elsevier, 1949) or alternatively that 5% nativecellulose are dissolved in an aqueous solution containing 8 to 10% NaOH(see T. Yamashiki, Journal of Applied Polymer Science, vol. 44, 691-698,1992).

On account of the different factors above, a person skilled in the artwas thus very naturally inclined to use weakly concentrated aqueoussodium hydroxide solutions for the regeneration of fibers made ofcellulose formate.

However, on continuing to increase the sodium hydroxide concentration inthe regenerating medium well beyond the abovementioned 5 to 6%, it hasbeen found, entirely surprisingly, that, beyond a certain concentrationthreshold, not only the phenomena of interfering dissolution disappearedbut also and especially that certain properties of the regenerated fiberwere very substantially improved, in particular the elongation at breakand the energy at break.

In other words, while a conventional regenerating medium (i.e. with alow concentration of sodium hydroxide) is certainly entirely sufficientto regenerate fibers made of cellulose formate, such a medium does not,however, make it possible to obtain fibers made of regenerated cellulosein accordance with the invention.

The method of the invention, for obtaining a fiber made of regeneratedcellulose in accordance with the invention, by regeneration of a fibermade of cellulose formate, is characterized in that the regeneratingmedium is a highly concentrated aqueous sodium hydroxide solution inwhich the sodium hydroxide concentration, recorded as Cs, is greaterthan 16% (% by weight).

Use is preferably made of a concentration Cs of greater than 18% and,even more preferably, a concentration of between 22% and 40%; this isbecause it has been found that such concentration ranges were, as ageneral rule, more particularly beneficial to the elongation at break ofthe regenerated fiber, the optimum concentration area being between 22%and 30%.

For the implementation of the regeneration method of the invention, thestarting material is preferably a fiber made of cellulose formate inaccordance with the invention having in particular an elongation atbreak ELb of greater than 6%.

The regeneration line consists, in concrete terms and conventionally, ofregeneration means, followed by washing means, themselves followed bydrying means. None of these devices is critical for the implementationof the invention and a person skilled in the art will know how to definethem without difficulty. The regeneration and washing means can consistin particular of baths, pipes, tanks or chambers in which theregenerating medium or the washing medium circulate. It is possible, forexample, to use chambers each equipped with two motorized rollers aroundwhich the fiber to be treated will be wound, this fiber then beingsprayed with the liquid medium employed (regenerating or washingmedium).

The residence times in the regeneration means should, of course, beadjusted so as substantially to regenerate the fibers made of formateand thus to verify the following relationship with respect to the finalregenerated fiber:

    0<D.sub.s <2.

A person skilled in the art will know how to adjust these residencetimes, which, depending on the specific conditions for implementation ofthe invention, can vary, for example, from 1 to 2 seconds up to 1 to 2tens of seconds.

The washing medium is preferably water. This is because, after the aboveregeneration operation, the fiber made of cellulose can be washed withits natural swelling medium, that is to say with water, the latterexhibiting the best washing efficiency. The water is used at roomtemperature or at a higher temperature, if necessary, in order toincrease the kinetics of washing. A neutralization agent for theunconsumed sodium hydroxide, for example formic acid, can optionally beadded to this washing water.

The drying means can consist, for example, of ventilated tunnel ovens,through which the washed fiber moves, or alternatively of heatingrollers on which the fiber is wound. The drying temperature is notcritical and can vary within a wide range, in particular from 80° C. to240° C. or more, as a function of the specific conditions forimplementation of the invention, in particular according to the rates ofpassage on the regeneration line. Use is preferably made of atemperature not exceeding 200° C.

At the outlet of the drying means, the fiber is removed from a receivingbobbin and its degree of residual moisture is monitored. The dryingconditions (temperature and duration) will preferably be adjusted sothat the degree of residual moisture is between 10% and 15%, morepreferably still of the order of 12% to 13%, by weight of dry fiber.

The washing and drying times necessary typically vary from a few secondsto a few tens of seconds, depending on the means employed and thespecific conditions for implementation of the invention.

During passage through the regeneration line, excessive tensions will,of course, be avoided in order not to damage the fiber, on the one hand,and not to lose, on the other hand, a significant part of the potentialelongation at break offered by the use of the regenerating medium whichis concentrated in sodium hydroxide. These tensions are generallydifficult to access within the different means employed themselves: theycan be monitored and measured at the inlet of these different means,using suitable tensiometers.

Thus, if it is desired to favor the elongation at break of theregenerated fiber, the tensile stresses at the inlet of the regenerationmeans, of the washing means and of the drying means will preferably bechosen to be less than 10 cN/tex, and more preferably still less than 5cN/tex.

Under actual industrial regeneration conditions, and in particular forhigh regeneration rates, the lower limits of these tensile stressesgenerally lie at approximately from 0.1 to 0.5 cN/tex, lower values notbeing realistic from an industrial viewpoint and even undesirable. Inparticular, it has been noticed that the mechanical properties of theregenerated fibers could be adjusted to a greater or lesser extent byvarying these tensile stresses.

The regeneration rate (recorded as Rr), that is to say the rate ofpassage of the fiber through the regeneration line, can vary fromseveral tens to several hundreds of meters per minute, for example up to400 or 500 m/min, or indeed more; advantageously, this rate Rr is atleast equal to 100 m/min, more preferably at least equal to 200 m/min.

Finally, the regeneration method of the invention is preferably employedin line and continuously with the spinning method of the invention, sothat the entire manufacturing line, from the extrusion of the solutionthrough the die to the drying of the regenerated fiber, isuninterrupted.

III. EXAMPLES OF THE IMPLEMENTATION OF THE INVENTION

The tests described hereinbelow can either be tests in accordance withthe invention or tests not in accordance with the invention.

III-1. FIBERS MADE OF CELLULOSE FORMATE

A) Fibers in Accordance with the Invention (Table 1):

A total of 14 spinning tests are carried out on fibers made of celluloseformate according to the spinning method of the invention and inaccordance in particular with the information provided in the aboveparagraphs II-1 and II-2.

The coagulation stage and the stage of neutral washing of the coagulatedfiber are both carried out in acetone.

Table 1 gives both the specific conditions for implementation of themethod of the invention and the properties of the fibers obtained.

The abbreviations and the units used in this Table 1 are as follows:

Test No.: number of the test (reference from A-1 to A-14);

N: number of filaments in the fiber;

C: concentration of cellulose in the spinning solution (% by weight);

DP: degree of polymerization of the cellulose in the spinning solution;

Rs: spinning rate (in m/min);

Tc: temperature of the coagulating medium (in ° C.);

Sr: degree of residual solvent in the fiber at the outlet of thecoagulation means (% by weight);

σ_(c) : tensile stress undergone by the fiber at the outlet of thecoagulation means (in cN/tex);

Yc: yarn count of the fiber (in tex);

Te: tenacity of the fiber (in cN/tex);

Mi: initial modulus of the fiber (in cN/tex);

ELb: elongation at break of the fiber (in %);

Eb: energy at break of the fiber (in J/g);

Ds: degree of substitution of the cellulose as formate groups in thefiber (in %).

In carrying out these tests, the following specific conditions areadditionally used:

all the spinning solutions are prepared from powdered cellulose (with aninitial water content equal to approximately 8% by weight and with adegree of polymerization of between 500 and 600), from formic acid andfrom orthophosphoric acid (each containing approximately 2.5% by weightof water);

these solutions contain (% by weight) from 16 to 22% cellulose, from 60to 65% phosphoric acid and from 18 to 19% formic acid (total), theinitial (formic acid/phosphoric acid) ratio by weight being equal toapproximately 0.30;

these solutions are optically anisotropic and contain a total of lessthan 10% water (% by weight);

the degree of substitution of the cellulose in the solutions is between40 and 45% for the solutions containing 16% by weight of cellulose andbetween 30 and 40% for the other, more concentrated solutions;

the dies contained 500 or 1000 capillaries of cylindrical shape, with adiameter of 50 or 65 μm;

the spinning temperatures are between 40 and 50° C.;

the SSF or SDF values are between 2 and 6 (between 2 and 4 for testsA-1, A-5 to A-9 and A-14; between 4 and 6 for the other tests);

the non-coagulating fluid layer is composed of a layer of air (thicknessvarying from 10 to 40 mm de pending on the tests);

the degree of phosphoric acid in the coagulating medium is stabilized ata level of less than 10% (% by weight of coagulating medium);

the temperature of the washing acetone (Tw) is always positive, between15 and 20° C.;

the fiber is dried at 70° C., by passing over heating rollers,supplemented by blowing nitrogen heated to 80° C.; the degree of acetoneat the outlet of the drying means is less than 0.5% (% by weight of dryfiber);

the degree of residual phosphoric acid on the completed fiber, i.e.washed and dried, is less than 0.1% (% by weight of dry fiber).

                                      TABLE 1                                     __________________________________________________________________________         N    C    Rs  Tc Sr                                                                              σ.sub.c                                                                     Yc Te  Mi  ELb                                                                              Eb Ds                                 TEST No. filaments % DP m/min ° C. % cN/tex tex cN/tex cN/tex %                                                   J/g %                            __________________________________________________________________________    A-1  1000 16                                                                              440                                                                              150 -30                                                                              40                                                                              0.7 213                                                                              53  1075                                                                              6.3                                                                              15.8                                                                             39                                 A-2 1000 20 430 150 -30 70 2.3 215 64 1405 6.4 18.7 36                        A-3 1000 22 430 150 -30 20 0.8 213 75 1720 6.7 23.8 33                        A-4 1000 20 430 150 -30 30 1.1 222 74 1540 7.2 24.7 37                        A-5 1000 16 450  55 -20 20 1.1 218 73 1565 8.2 29.5 41                        A-6 1000 16 440  55 -20 20 0.8 220 63 1205 8.7 26.2 42                        A-7 1000 16 440 150 -30 35 0.7 224 48  955 6.5 14.6 42                        A-8 1000 16 440 150 -30 35 2.3 217 57 1305 6.9 18.7 40                        A-9 1000 16 430  55 -30 10 9.4 213 73 1760 6.4 22.2 42                         A-10  500 22 420 150 -30 30 1.0 115 70 1305 6.5 20.4 32                       A-11  500 22 420 150 -15 30 1.0 117 76 1365 6.9 23.0 32                       A-12  500 22 420 150 -10 30 1.0 118 71 1330 6.8 21.3 32                       A-13  500 22 420 150  0 30 1.0 122 67 1375 6.6 20.3 32                        A-14  500 16 450 150 -30 35 4.5 112 65 1295 6.5 19.6 42                    __________________________________________________________________________

On reading Table 1, it is noted in particular that, with the exceptionof test A-13, the temperature Tc of the coagulation acetone is alwaysnegative, less than -10° C. in the majority of the cases.

The DP of the cellulose in the solution is between 400 and 450, whichshows in particular a low depolymerization after solubilization.

In addiction, it is found that, for all the test in Table 1, at leastone of the following preferred conditions is verified:

    Sr<100%; σ.sub.c <5 cN/tex,

and that these two relationships are simultaneously verified in themajority of cases.

In an even more preferred way , the two following relationships aresimultaneously verified:

    Sr<50%; σ.sub.c <2 cN/tex.

Moreover, the spinning rates are high, since they are for most partequal to 150 m/min.

All the mechanical properties shown in Table 1 are mean valuescalculated with respect to 10 measurements, with the exception of theyarn count (mean with respect to

3 measurements), the standard deviation with respect to the mean (as %of this mean) generally being between 1 and 2.5%.

On reading Table 1, it is found that all the fibers verify the followingrelationships:

Ds≧2;

Te>45;

Mi>800;

ELb>6;

Eb>13.5.

Preferably, for the fibers made of cellulose formate of the invention,the Ds values are between 25 and 50%. It is found that, in theseexamples, they are all between 30 and 45%: in practice, they areidentical to the values of degrees of substitution measured on thecorresponding spinning solutions.

Preferably, their elongation at break ELb is greater than 7% (ExamplesA-4 to A-6), more preferably still greater than 8% (Examples A-5 andA-6).

Moreover, these fibers of Table 1 verify, for the most part, thefollowing preferred relationships:

Te>60; Mi>1200; Eb >20.

More preferably still, at least one of the following relationships isverified:

Te>70; Mi>1500; Er>25.

For all the examples in Table 1, it is additionally found that thefollowing relationship is verified:

Mi<1800.

However, particularly high initial modulus values, for example ofbetween 1800 and 2200 cN/tex, or even more, are also accessible withrespect to the fibers made of formate in accordance with the invention,normally to the detriment of the elongation at break, by adjusting theparameters of the spinning method according to the invention. This canbe achieved in particular by increasing the tensile stresses on thespinning line, for example at the outlet of the coagulation means,during the washing or alternatively during the drying of the fiber; ithas also been observed that the use of relatively high concentrations C,in particular of between 24 and 30%, is favorable to the production ofvery high initial moduli and tenacities.

B) Fibers not in Accordance with the Invention (Table 2):

5 spinning tests (referenced from B-1 to B-5) are carried out on fibersmade of cellulose formate according to a spinning method not inaccordance with the invention.

The general and specific conditions used for the spinning are the sameas those used for the fibers in the above Table 1, apart from oneexception: the stage of neutral washing of the coagulated fiber iscarried out with water (as in the abovementioned Application WO85/05115) and not with acetone. This washing water is process water at atemperature in the region of 15° C. Moreover, the fibers contain from250 to 1000 filaments.

Table 2 gives both the specific conditions for implementation of themethod of the invention and the properties of the fibers obtained. Theabbreviations and the units used in this Table 2 are the same as for theabove Table 1.

                                      TABLE 2                                     __________________________________________________________________________         N    C    Rs  Tc Sr σ.sub.c                                                                     Yc Te  Mi  ELb                                                                              Eb Ds                                TEST No. filaments % DP m/min ° C. % cN/tex tex cN/tex cN/tex %                                                    J/g %                           __________________________________________________________________________    B-1  500  16                                                                              450                                                                              200 -20                                                                              60 0.9 110                                                                              67  2050                                                                              5.2                                                                              18.9                                                                             42                                B-2 1000  22 420 150 -30 25 0.8 220 78 2150 5.1 20.6 32                       B-3 500 16 450 200 -30 60 0.5 110 60 1940 4.4 13.9 40                         B-4 250 22 450 150 -20 120  1.0  56 83 2810 4.0 17.5 33                       B-5 750 16 420 200 -30 60 0.9 168 59 1685 4.7 14.6 42                       __________________________________________________________________________

It is noted that these fibers in Table 2, spun according to the methodtaught by the abovementioned Application WO 85/05115, can exhibitentirely advantageous characteristics of tenacity and of initialmodulus; in particular, after a conventional regeneration stageaccording to the prior art (weakly concentrated aqueous NaOH solution),they can be converted to regenerated fibers possessing very hightenacities (110 to 120 cN/tex, or even more) combined with very highinitial modulus values (3000 to 3500 cN/tex, or indeed more).

Nevertheless, none of these fibers in Table 2 is in accordance with theinvention, the following relationship not being verified:

ELb>6.

III-2. FIBERS MADE OF REGENERATED CELLULOSE

A) Fibers in Accordance with the Invention (Table 3):

A total of 23 regeneration tests are carried out on fibers made ofcellulose formate in accordance with the regeneration method of theinvention, according to the information provided in the above paragraphII-3.

All these regeneration tests are carried out in line and continuouslywith the spinning operation, the latter being carried out in accordancewith the spinning method of the invention: in particular, thecoagulation stage and the stage of neutral washing of the coagulatedfiber are both carried out in acetone.

The regenerating medium is an aqueous sodium hydroxide solution, theconcentration Cs of which is in all cases greater than 16%.

Table 3 gives both specific conditions for the implementation of themethod of the invention and the properties of the fibers obtained.

The abbreviations and the units used in this Table 3 are as follows:

Test No.: number of the test (referenced from C-1 to C-23);

N: number of filaments in the regenerated fiber;

Cs: concentration of sodium hydroxide in the regenerating medium (% byweight);

Rr: rate of regeneration (in m/min);

Y_(C) : yarn count of the fiber (in tex);

T_(E) : tenacity of the fiber (in cN/tex);

M_(I) : initial modulus of the fiber (in cN/tex);

EL_(B) : elongation at break of the fiber (in %);

E_(B) : energy at break of the fiber (in J/g).

In carrying out these tests, the following specific conditions areadditionally used:

the starting fibers made of cellulose formate, a sample of which (a fewtens of meters) has been systematically removed at the outlet of thespinning means, in order to monitor their mechanical properties, are allin accordance with the invention; in particular, they all possess anelongation at break of greater than 6%;

the regenerating medium used is at room temperature (approximately 20°C.);

the regeneration, washing and drying means are composed of chambersequipped with motorized rollers on which the fiber to be treated will bewound;

as the regeneration is carried out in line and continuously with thespinning, the rate of regeneration Rr shown in Table 3 (from 55 to 200m/min) is thus equal to the spinning rate Rs;

washing is carried out with process water at a temperature ofapproximately 15° C.;

the washed fiber is dried on heating rollers, at different temperaturesvarying from 80° C. to 240° C., according to the specific scheme below:from 80° C. to 120° C. for tests C-2, C-3, C-5, C-10 and C-17; at 240°C. for test C-11; from 160° C. to 190° C. for the other tests;

the tensile stresses measured at the inlet of the regeneration, washingand drying means are always less than 10 cN/tex, in the majority ofcases less than 5 cN/tex, except for tests C-7, C-9 and C-15, where atension equal to or greater than 5 cN/tex was measured at the inlet ofat least one of the above means; these tensile stresses are lower than 2cN/tex at each inlet of the three means stated above (regeneration,washing and drying) for a large number of tests: C-2 to C-5, C-10 toC-11, C-13 to C-14 and C-16 to C-23;

the residence times in the regeneration means are of the order of 15 s,as in the washing means, whereas they are of the order of 10 s in thedrying means;

at the outlet of the drying means, the fibers exhibit a degree ofresidual moisture of the order of 12% to 13% (% by weight of dry fiber).

                  TABLE 3                                                         ______________________________________                                        TEST N        Cs    Rr    Y.sub.C                                                                            T.sub.E                                                                             M.sub.I                                                                             EL.sub.B                                                                           E.sub.S                         No. filaments % m/min tex cN/tex cN/tex % J/g                               ______________________________________                                        C-1  500      18    150   92   100   2295  6.8  33.3                            C-2  500 20 200 91 79 2020 6.7 26.5                                           C-3  1000 24 55 186 73 1815 6.2 22.0                                          C-4  1000 24 55 183 82 1775 8.4 33.9                                          C-5  500 30 200 90 81 1780 7.8 30.6                                           C-6  1000 30 150 176 85 1905 7.2 29.9                                         C-7  1000 30 150 179 104 2360 7.2 36.1                                        C-8  500 30 150 90 97 2080 7.3 34.6                                           C-9  500 30 150 90 98 2170 7.0 33.4                                           C-10 500 30 150 93 83 1990 7.3 30.3                                           C-11 500 30 150 90 89 2075 7.4 32.6                                           C-12 500 30 150 98 99 2335 6.9 33.7                                           C-13 500 30 200 90 81 1690 7.9 30.8                                           C-14 1000 30 200 180 73 1565 7.7 26.9                                         C-15 1000 30 150 180 82 1845 7.7 33.9                                         C-16 1000 30 150 178 97 2245 7.3 34.5                                         C-17 1000 40 200 90 81 2055 6.9 28.4                                          C-18 500 30 200 89 108 2540 6.6 34.6                                          C-19 500 30 200 136 99 2270 7.2 35.0                                          C-20 500 30 200 181 90 2000 7.6 33.1                                          C-21 500 30 200 91 107 2580 6.5 34.1                                          C-22 500 30 200 85 102 2450 6.8 34.3                                          C-23 500 30 200 97 87 2210 6.8 30.6                                         ______________________________________                                    

A measurement of the degree of substitution, as indicated in paragraphI-2.2, has shown that all the fibers in Table 3 have a D_(s) value ofbetween 0 and 2%, in the great majority of cases between 0.1 and 1%.

As for the preceding results, all the mechanical properties shown inTable 3 are mean values calculated with respect to 10 measurements, withthe exception of the yarn count (mean with respect to 3 measurements),the standard deviation with respect to these different means (as % ofthe mean) generally being between 1 and 2.5%.

It is found that the regenerated fibers in Table 3 verify all thefollowing relationships:

T_(E) >60;

M_(I) >1000;

EL_(B) >6;

E_(B) >17.5.

Preferably, their elongation at break EL_(B) is greater than 7%(Examples C-4 to C-11, C-13 to C-16, C-19 and C-20), more preferablystill greater than 8% (Example C-4).

The best value of elongation at break (EL_(B) =8.4% for test C-4) has inparticular been obtained by spinning and regeneration in line of asolution containing 16% by weight of cellulose for which the DP wasequal to approximately 420. The sample of corresponding fiber made offormate, removed at the spinning outlet in order to measure themechanical properties, showed the following properties:

Ds=40; Te=60; Mi=1290; ELb=8.4; Eb=25.3.

Moreover, the great majority of the fibers in Table 3 verify thefollowing relationships:

T_(E) >80; M_(I) >1500; E_(B) >25,

a great number of them verifying at least one of the followingrelationships:

T_(E) >100; M_(I) >2000; E_(B) >30.

Particularly high tenacities (equal to or greater than 100 cN/tex) arerecorded in particular in the case of tests C-1, C-7, C-18, C-21 andC-22, combined with high values of elongation and of energy at break,indeed even with high values of initial modulus, greater than 2400cN/tex in the case of tests C-18, C-21 and C-22.

For all the examples in Table 3, it is additionally found that thefollowing relationship is verified:

M_(I) <2600.

However, particularly high initial modulus values, for example ofbetween 2600 and 3000 cN/tex, are also accessible with respect to theregenerated fibers in accordance with the invention, normally to thedetriment of the elongation at break, by adjusting the parameters of theregeneration method according to the invention. This can be achieved inparticular by increasing the tensile stresses on the regeneration lineor alternatively by selecting starting fibers (made of celluloseformate) which already exhibit particularly high initial modulus values,for example between 1800 and 2200 cN/tex.

While, for the majority of the examples in Table 3, the filament yarncount (yarn count of the fiber Y_(c) divided by the number N offilaments) is equal to approximately 1.8 dtex (decitex) (the commonestfilament yarn count for cellulose fibers), the latter can vary to alarge extent, for example from 1.4 dtex to 4.0 dtex, or indeed more, byadjusting, in a known way, the spinning conditions. By way of example,the regenerated fibers in tests C-19 and C-20 possess, respectively, afilament yarn count of 2.9 dtex and of 3.6 dtex. Generally, an increasein the elongation at break EL_(B), combined with a decrease in thetenacity T_(E) and in the initial modulus M_(I), has been observed whenthe filament yarn count increases.

B) Fibers not in Accordance with the Invention (Table 4):

A total of 9 regeneration tests are carried out on fibers made ofcellulose formate (referenced from D-1 to D-9) according to aregeneration method not in accordance with the invention.

The regeneration conditions are the same as those used for the fibers inaccordance with the invention in the above Table 3, apart from oneexception: the regenerating medium is an aqueous sodium hydroxidesolution in which the sodium hydroxide concentration Cs is at most equalto 16%.

Table 4 gives both the specific conditions for the implementation of themethod of the invention and the properties of the fibers obtained. Theabbreviations and the units used in this Table 4 are the same as for theabove Table 3.

                  TABLE 4                                                         ______________________________________                                        TEST N        Cs    Rr    Y.sub.C                                                                            T.sub.E                                                                             M.sub.I                                                                             EL.sub.B                                                                           E.sub.S                         No. filaments % m/min tex cN/tex cN/tex % J/g                               ______________________________________                                        D-1  1000     1     100   184  85    2280  5.6  23.6                            D-2 250 1.5 100 46 76 2600 4.8 17.9                                           D-3 500 3 150 98 84 2315 5.2 21.7                                             D-4 500 6 150 96 67 1895 4.4 14.3                                             D-5 500 12 150 108 73 1975 5.0 17.8                                           D-6 500 16 200 93 63 1750 5.9 18.6                                            D-7 500 1 200 90 103 2750 5.6 29.0                                            D-8 500 1.5 200 95 107 3050 4.8 25.3                                          D-9 500 1.7 200 87 111 2970 5.0 27.4                                        ______________________________________                                    

All the fibers obtained are indeed regenerated, insofar as, aftermonitoring, the values for degree of substitution D_(s) are always lessthan 2%, more specifically between 0.1% and 1.0%.

These fibers in Table 4 can exhibit particularly high characteristics oftenacity and of initial modulus (see in particular D-7 to D-9) but it isfound that none of them is in accordance with the invention, thefollowing relationship not being verified:

EL_(B) >6.

In Examples D-4 and D-5 (Cs=6% and 12%), a partial dissolution at thesurface of the filaments was observed, resulting in the presence ofbonded filaments and in a poor general condition of the fiber, resultingin very great difficulties in carrying out a neutral washing. In ExampleD-6, the same phenomena were encountered but to a lesser extent: this isat the limits of the method of the invention (Cs=16%) and, inparticular, an elongation at break very close to 6% is recorded.

A comparision of Examples D-3 and C-12 (Table 3) proves to be quiteinteresting, insofar as the regeneration operations were carried out onthe same fiber made of cellulose formate and, with the exception of thesodium hydroxide concentration in the regenerating medium (3% for testD-3, 30% for test C-12), under specific conditions which are strictlyidentical.

In fact, it is found that, with respect to a conventional regenerationwith a weakly concentrated sodium hydroxide solution (test D-3), themethod of the invention (test C-12) made it possible to verysubstantially improve the values of tenacity (increase of 18%), ofelongation at break (increase of 33%) and of energy at break (increaseof 55%), without significantly modifying the initial modulus value.

All the fibers in the above Tables 1 to 4, made of cellulose formate ormade of regenerated cellulose, whether they are or are not in accordancewith the invention, exhibit a typical structure and a typical morphologyfor products spun from a liquid crystal solution, as described inparticular in the original application WO 85/05115.

In particular, when their filaments are studied with an opticalmicroscope or a scanning electron microscope, a morphology is observedsuch that each filament is composed, at least in part, of layers fittedinside one another surrounding the axis of the filament. In addition, itis found that in each layer, in general, the optical direction and thecrystallization direction vary virtually periodically along the axis ofthe filament. Such a structure or morphology is commonly described inthe literature under the name of "banded structure".

C) Other Properties of the Fibers Made of Regenerated Cellulose inAccordance with the Invention--Use in Tires:

In addition to the improved mechanical properties stated above, thefibers made of regenerated cellulose of the invention exhibit numerousother advantages when they are compared with the fibers described in theabovementioned original application WO 85/05115, on the one hand, andwith conventional fibers of the rayon type, on the other hand.

C-1. Comparison with Fibers Made of Regenerated Cellulose According toWO 85/05115:

Compared with the fibers described in the original application WO85/05115, the fibers of the invention in particular exhibit a verysubstantially improved resistance to fatigue, both in laboratory testsand when the tire is run.

Endurance with Respect to Compression (laboratory test):

For technical fibers, intended in particular to reinforce tirestructures, the resistance to fatigue can be analyzed by subjectingassemblies of these fibers to various known laboratory tests, inparticular to the fatigue test known under the name of Disk Fatigue Test(see, for example, U.S. Pat. No. 2,595,069 and ASTM Standard D 885-591,revised 67T).

This test, well known to a person skilled in the art (see, for example,U.S. Pat. No. 4,902,774), consists essentially in incorporating pliedyarns of the test fibers, treated with an adhesive beforehand, in rubberblocks and then, after curing, in fatiguing the rubber test specimensthus formed by compression, between two rotating disks, a very largenumber of cycles (for example, between 100,000 and 1,000,000 cycles).After fatigue, the plied yarns are extracted from the test specimens andtheir residual breaking strength is compared with the breaking strengthof control plied yarns extracted from non-fatigued test specimens.

The fibers of the invention, compared with the fibers of the originalapplication WO 85/05115, systematically show a markedly improvedendurance in the Disk Fatigue Test.

By way of example, fibers according to the invention exhibiting apreferred elongation at break of greater than 7% and fibers according toApplication WO 85/05115, all having an elongation at break of less than5%, were assembled in order to form plied yarns (of type "A" and "B",respectively) having the same formula 180×2 (tex) 420/420 (t/m).

In a known way, such a formula means that each plied yarn is composed oftwo spun yarns (multi-filament fibers), each having a yarn count of 180tex before twisting, which are first individually twisted at 420 t/m inone direction, during a first stage, and are then both twisted togetherat 420 t/m in the reverse direction, during a second stage. For such aplied yarn, the helical angle is approximately 27° and the twistcoefficient (or alternatively twist factor) K is approximately 215,with:

    K=Twist of the plied yarn (in t/m)×[Yarn count of the plied yarn (in tex)/1520].sup.1/2.

(cellulose relative density: 1.52)

Several plied yarns of the "A" type (according to the invention) and ofthe "B" type (according to WO 85/05115) were subjected to the above DiskFatigue Test (6 hours at 2700 cycles/min, with a maximum degree ofcompression of the test specimen of approximately 16% in each cycle);the declines in breaking strength which follow were recorded on theplied yarns extracted (given as relative values, with a base of 100 forthe maximum decline recorded on a plied yarn of the "B" type):

type "A" plied yarn: 25 to 40;

type "B" plied yarn: 70 to 100.

The resistance to fatigue of the regenerated fibers of the invention isthus markedly improved, by a factor of two to three on average, withrespect to the regenerated fibers of the original application WO85/05115.

Endurance in Tires:

The ability of technical fibers to reinforce tires can be analyzed, in aknown way, by reinforcing a rubber ply with plied yarns of the testfibers, which have been treated with adhesive beforehand, byincorporating the fabric thus formed in a tire structure, for example ina carcass ply, and by then subjecting the tire, thus reinforced, to arunning test.

Such running tests are widely known to a person skilled in the art; theycan, for example, be carried out on automatic machines which make itpossible to vary a large number of parameters (pressure, load,temperature, and the like) during the running. After running, the pliedyarns are extracted from the tested tire and their residual breakingstrength is compared with that of control plied yarns extracted fromcontrol tires which have not been subjected to running.

It was found that the fibers of the invention, when they are used toreinforce a radial tire carcass, show an endurance which is markedlyimproved with respect to the fibers according to WO 85/05115. Inparticular, it has been observed that, where fibers according to theprior art did not show resistance (failure of the plied yarns of the "B"type above), due to particularly severe running conditions, the fibersof the invention (plied yarns of the "A" type above) showed virtually nodecline, even after several tens of thousands of kilometers.

C-2. Comparison with Conventional Fibers of the Rayon Type:

In addition to their markedly higher elongational mechanical properties,the regenerated fibers of the invention exhibit other entirelyadvantageous characteristics in comparison with conventional rayonfibers.

Resistance to Moisture:

The resistance to moisture of cellulose fibers can be analyzed usingvarious known tests, a simple test consisting, for example, incompletely soaking the fibers in a water bath for a predetermined timeand in then measuring the breaking strength of the fibers in the wetstate, by immediately subjecting them to tension at the outlet of thewater bath after having simply drained them dry.

After storing for 24 hours in water at room temperature, is is foundthat the breaking strength in the wet state for the fibers of theinvention represents 80 to 90%, depending on the case, of the nominalbreaking strength (i.e. in the dry state, measured as indicated inparagraph I-4). For rayon fibers, it represents no more thanapproximately 60% of the nominal breaking strength.

The fibers of the invention are thus markedly less sensitive to moisturethan conventional rayon fibers; they exhibit a better dimensionalstability in a moist environment.

Mechanical Properties with Respect to Plied Yarns:

The fibers of the invention can be assembled, as described above, inorder to form reinforcing assemblies with high or very high mechanicalproperties, in particular plied yarns, the construction of which can beadapted to a very large extent according to the envisaged application.It is known, for example, that an increase in the twist, i.e. in thehelical angle, generally improves the endurance of the plied yarn,increases its elongation at break, while, however, being harmful to itstenacity and to its extensional modulus.

Even for very high twists, corresponding, for example, to a helicalangle of the order of 29-30°, which confer excellent enduranceproperties on the plied yarns, the fibers of the invention, in thetwisted state, possess a tenacity which is still superior to thetenacity of non-twisted rayon fibers.

By way of example, the plied yarns in accordance with the invention,prepared according to known twisting methods from the fibers of theinvention, exhibit, when the helical angle of the plied yarn is variedfrom 20 up to 30 degrees, a tenacity which can vary from 75-80 cN/tex upto 45-50 cN/tex, for example a tenacity of the order of 58-66 cN/tex fora helical angle of 23-24° (K=approximately 180) or of 53-57 cN/tex for ahelical angle of 26-27° (K=approximately 215), and an elongation atbreak which can reach values of approximately 10%, if not more.

Thus, the tenacities of the plied yarns in accordance with theinvention, with an equivalent twist (same helical angle), are generallymuch greater than the tenacities with respect to plied yarns which canbe obtained from fibers of the rayon type, the tenacity of whichscarcely exceeds, in a known way, 45-50 cN/tex before twisting. It willthus be possible to use a smaller amount of them in articles commonlyreinforced by conventional rayon fibers.

Endurance in Tires:

For actual running conditions, employed on private vehicles equippedwith tires of size 165/70 R 13, it was unexpectedly found that fibers ofthe invention (despite a markedly more rigid and more crystallinestructure, since they result from a liquid crystal phase) displayedthroughout the running tests (for example, monitoring every 5000 km from20,000 to 80,000 km) an endurance identical to that of a conventionalrayon fiber, for an identical plied yarn construction.

Extensional Moduli:

The fibers of the invention, the primary characteristic of which is animproved elongation at break, have an initial modulus which remainsaltogether high (for example, 1500 to 2600 cN/tex approximately in Table3), in all cases very markedly higher than that of conventional rayonfibers (1000 cN/tex approximately, in a known way).

This superiority of the fibers of the invention in terms of modulus,which is, of course, encountered with respect to the reinforcingassemblies of these fibers, can be altogether advantageous for articlescommonly reinforced by conventional technical rayon fibers by offeringsuch articles the possibility of an improved dimensional stability: thisis because, for the same variation Δ(F) in the load or force "F" whichis exerted on an assembly of each type, the assembly in accordance withthe invention will undergo a markedly smaller variation Δ(EL) in lengthor in elongation "EL".

In conclusion, a comparison of the results of the invention with thosedescribed in Application WO 85/05115, both for fibers made of celluloseformate and for fibers made of regenerated cellulose, shows that theinvention has made it possible not only to very substantially increasethe values of elongation at break, which are more than doubled incertain cases, but also to maintain the tenacity values at a very highlevel, indeed even to improve them in numerous cases.

The advantage of such a result must be particularly emphasized.

The improvement introduced by the invention does not consist of a simpleshift toward another optimum in a given [tenacity-elongation at break]combination, with an energy at break which remains substantially thesame (total area under the Force-Elongation stress curve remainingsubstantially constant); it consists, in fact, of a very substantialimprovement in any [tenacity-elongation at break] combination, making itpossible, as it were, to "extend" the Force-Elongation curves obtainedfor the fibers of the original application WO 85/05115 and thus toobtain a very markedly improved energy at break (increased area underthe Force-Elongation curve).

Of course, the invention is not limited to the examples described above.

Thus, for example, different constituents can optionally be added to thebasic constituents described above (cellulose, formic acid, phosphoricacid, acetone and sodium hydroxide), without the spirit of the inventionbeing modified.

Thus, the term "cellulose formate" used in this document covers thecases where the hydroxyl groups of the cellulose are substituted bygroups other than formate groups, in addition to the latter, for exampleester groups, in particular acetate groups, the degree of substitutionof the cellulose as these other groups preferably being less than 10%.

The additional constituents, preferably chemically nonreactive with thebasic constituents, can be, for example, plasticizers, sizing agents,dyes or polymers other than cellulose which are optionally capable ofbeing esterified during the preparation of the solution. They can alsobe various additives which make it possible, for example, to improve thespinnability of the spinning solutions, the use properties of the fibersobtained or the adhesiveness of these fibers to a rubber matrix.

The invention also covers the cases where use is made of a die composedof one or more non-cylindrical capillaries with various shapes, forexample of a single capillary in the form of a slit, the term "fiber"used in the description and the claims then having to be understood in amore general sense which can include, in particular, the case of a filmmade of cellulose formate or of a film made of regenerated cellulose.

We claim:
 1. Fiber made of cellulose formate, characterized by thefollowing relationships:Ds between 25 and 50; Te>45; Mi>800; ELb>6;Eb>13.5;Ds being the degree of substitution of the cellulose an formategroups (in %), Te being its tenacity in cN/tex, Mi being its initialmodulus in cN/tex, ELb being its elongation at break in % and Eb beingits energy at break in J/g.
 2. A method for spinning a solution ofcellulose formate in a solvent based on phosphoric acid, according tothe so-called dry-jet-wet spinning method, in order to obtain a fibermade of cellulose formate, characterized by the followingrelationships:Ds≧2; Te>45; Mi>800; ELb>6; Eb>13.5,Ds being the degree ofsubstitution of the cellulose as formate groups (in %), Te being thetenacity in cN/tex, Mi being its initial modulus in cN/tex, ELb beingits elongation at break in % and Eb being its energy at break in J/g,characterized in that the stage of coagulation of the fiber and thestage of neutral washing of the coagulated fiber are both carried out inacetone.
 3. Method according to claim 2, characterized in that thetemperature of the coagulation acetone is negative and in that thetemperature of the washing acetone in positive.
 4. Method according toclaim 3, characterized in that the following relationships exist:Tc<-10°C.; Tw>+10° C.,Tc being the temperature of the coagulation acetone andTw being the temperature of the washing acetone.
 5. Method according toclaim 2, characterized in that at least one of the followingcharacteristics is verified:a) the degree of residual solvent in thefiber, at the outlet of the coagulation means Sr), is less than 100% byweight of dry fiber; b) the tensile stress undergone by the fiber, atthe outlet of the coagulation means (σ_(c)), is less than 5 cN/tex. 6.Method according to claim 5, characterized by the followingrelationships:Sr<50%; σ_(c) <2 cN/tex.
 7. The method according to claim2 wherein the fiber is characterized by the followingrelationship:ELb>7.
 8. The method according to claim 2 wherein the fiberis characterized by the following relationship:ELb>8.
 9. The methodaccording to claim 2 wherein the fiber is characterized by the followingrelationships:Te>60; Mi>1200; Eb>20.
 10. The method according to claim 2wherein the fiber is characterized by at least one of the followingrelationships:Te>70; Mi>1500; Eb>25.
 11. Method according to claim 7wherein the fiber is further characterized by the followingrelationships:Te>60; Mi>1200; Eb>20.
 12. Method according to claim 8wherein the fiber is further characterized by the followingrelationships:Te>60; Mi>1200; Eb>20.
 13. Method according to claim 7wherein the fiber is further characterized by at least one of thefollowing relationships:Te>70; Mi>1500; Eb>25.
 14. Method according toclaim 8 wherein the fiber is further characterized by at least one ofthe following relationships:Te>70; Mi>1500; Eb>25.
 15. Method accordingto claim 3, characterized in that at least one of the followingcharacteristics is verified:a) the degree of residual solvent in thefiber, at the outlet of the coagulation means (recorded as Sr), is lessthan 100% by weight of dry fiber; b) the tensile strength undergone bythe fiber, at the outlet of the coagulation means (recorded as σ_(c)) isless than 5cN/tex.
 16. Method according to claim 4, characterized inthat at least one of the following characteristics is verified:c) thedegree of residual solvent in the fiber, at the outlet of thecoagulation means (recorded as Sr), is less than 100% by weight of dryfiber; d) the tensile strength undergone by the fiber, at the outlet ofthe coagulation means (recorded as σ_(c)) is less than 5cN/tex. 17.Fiber according to claim 1, characterized by the followingrelationships:ELb>7.
 18. Fiber according to claim 17, characterized bythe following relationship:ELb>8.
 19. Fiber according to claim 1,characterized by the following relationships:Te>60; Mi>1200; Eb>20. 20.Fiber according to claim 19, characterized by at least one of thefollowing relationships:Te>70; Mi>1500; Eb>25.